| 1. |
- Schönhense, G., et al.
(författare)
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Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens
- 2021
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 92:5
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Tidskriftsartikel (refereegranskat)abstract
- The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for E-kin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 mu m above the sample surface for E-kin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at E-kin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm(2) (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm(2), it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at E-kin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.
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| 2. |
- Chernenkaya, A., et al.
(författare)
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Microscopic origin of the charge transfer in single crystals based on thiophene derivatives : A combined NEXAFS and density functional theory approach
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:3
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the charge transfer mechanism in single crystals of DTBDT-TCNQ and DTBDT-F4TCNQ (where DTBDT is dithieno[2,3-d;2′,3′-d′] benzo[1,2-b;4,5-b′]dithiophene) using a combination of near-edge X-ray absorption spectroscopy (NEXAFS) and density functional theory calculations (DFT) including final state effects beyond the sudden state approximation. In particular, we find that a description that considers the partial screening of the electron-hole Coulomb correlation on a static level as well as the rearrangement of electronic density shows excellent agreement with experiment and allows to uncover the details of the charge transfer mechanism in DTBDT-TCNQ and DTBDT-F4 TCNQ, as well as a reinterpretation of previous NEXAFS data on pure TCNQ. Finally, we further show that almost the same quality of agreement between theoretical results and experiment is obtained by the much faster Z+1/2 approximation, where the core hole effects are simulated by replacing N or F with atomic number Z with the neighboring atom with atomic number Z+1/2.
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| 3. |
- Medjanik, K., et al.
(författare)
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Near-Edge x‑ray absorption fine structure investigation of the quasi-One-Dimensional organic conductor (TMTSF)2PF6
- 2016
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 120:43, s. 8574-8583
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Tidskriftsartikel (refereegranskat)abstract
- We present high-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the P L2/3 edges, F K edge, C K edge, and Se M2/3 edges of the quasi-one-dimensional (1D) conductor and superconductor (TMTSF)2PF6. NEXAFS allows probing the donor and acceptor moieties separately; spectra were recorded between room temperature (RT) and 30 K at normal incidence. Spectra taken around RT were also studied as a function of the angle (θ) between the electric field of the X-ray beam and the 1D conducting direction. In contrast with a previous study of the S L2/3-edges spectra in (TMTTF)2AsF6, the Se M2/3 edges of (TMTSF)2PF6 do not exhibit a well-resolved spectrum. Surprisingly, the C K-edge spectra contain three well-defined peaks exhibiting strong and nontrivial θ and temperature dependence. The nature of these peaks as well as those of the F K-edge spectra could be rationalized on the basis of first-principles DFT calculations. Despite the structural similarity, the NEXAFS spectra of (TMTSF)2PF6 and (TMTTF)2AsF6 exhibit important differences. In contrast with the case of (TMTTF)2AsF6, the F K-edge spectra of (TMTSF)2PF6 do not change with temperature despite stronger donor−anion interactions. All these features reveal subtle differences in the electronic structure of the TMTSF and TMTTF families of salts.
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| 4. |
- Schönhense, G., et al.
(författare)
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Time-of-flight photoelectron momentum microscopy with 80-500 MHz photon sources : Electron-optical pulse picker or bandpass pre-filter
- 2021
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Ingår i: Journal of Synchrotron Radiation. - 0909-0495. ; 28, s. 1891-1908
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Tidskriftsartikel (refereegranskat)abstract
- The small time gaps of synchrotron radiation in conventional multi-bunch mode (100-500 MHz) or laser-based sources with high pulse rate (∼80 MHz) are prohibitive for time-of-flight (ToF) based photoelectron spectroscopy. Detectors with time resolution in the 100 ps range yield only 20-100 resolved time slices within the small time gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron-optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows electron-optical 'pulse-picking' with any desired repetition period. Aberration-free momentum distributions have been recorded at reduced pulse periods of 5 MHz (at MAX II) and 1.25 MHz (at BESSY II). The approach is compared with two alternative solutions: A bandpass pre-filter (here a hemispherical analyzer) or a parasitic four-bunch island-orbit pulse train, coexisting with the multi-bunch pattern on the main orbit. Chopping in the time domain or bandpass pre-selection in the energy domain can both enable efficient ToF spectroscopy and photoelectron momentum microscopy at 100-500 MHz synchrotrons, highly repetitive lasers or cavity-enhanced high-harmonic sources. The high photon flux of a UV-laser (80 MHz, <1 meV bandwidth) facilitates momentum microscopy with an energy resolution of 4.2 meV and an analyzed region-of-interest (ROI) down to <800 nm. In this novel approach to 'sub-μm-ARPES' the ROI is defined by a small field aperture in an intermediate Gaussian image, regardless of the size of the photon spot.
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| 5. |
- Lytvynenko, Y., et al.
(författare)
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Control of the asymmetric band structure in Mn2Au by a ferromagnetic driver layer
- 2023
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 108:10
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Tidskriftsartikel (refereegranskat)abstract
- Hard x-ray angle-resolved photoemission spectroscopy reveals the momentum-resolved band structure in an epitaxial Mn2Au(001) film capped by a 2-nm-thick ferromagnetic permalloy layer. By magnetizing the permalloy capping layer, the exceptionally strong exchange bias aligns the Néel vector in the Mn2Au(001) film accordingly. Uncompensated interface Mn magnetic moments in Mn2Au were identified as the origin of the exchange bias using x-ray magnetic circular dichroism in combination with photoelectron emission microscopy. Using time-of-flight momentum microscopy, we measure the asymmetry of the band structure, E(k)≠E(-k), in Mn2Au resulting from the homogeneous orientation of the Néel vector. Comparison with theory shows that the Néel vector, determined by the magnetic moment of the top Mn layer, is antiparallel to the permalloy magnetization. The experimental results demonstrate that hard x-ray photoemission spectroscopy can measure the band structure of epitaxial layers beneath a metallic capping layer and corroborate the asymmetric band structure in Mn2Au that was previously inferred only indirectly.
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| 6. |
- Schlueter, C., et al.
(författare)
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New HAXPES Applications at PETRA III
- 2018
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Ingår i: Synchrotron Radiation News. - : Informa UK Limited. - 0894-0886 .- 1931-7344. ; 31:4, s. 29-35
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Tidskriftsartikel (refereegranskat)
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| 7. |
- Schlueter, C., et al.
(författare)
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The new dedicated HAXPES beamline P22 at PETRAIII
- 2019
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Ingår i: 13th International Conference on Synchrotron Radiation Instrumentation (SRI2018). - : American Institute of Physics (AIP). - 9780735417823
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Konferensbidrag (refereegranskat)abstract
- A new undulator beamline (P22) for hard X-ray photoelectron spectroscopy (HAXPES) was built at PETRA III (DESY, Hamburg) to meet the increasing demand for HAXPES-based techniques. It provides four special instruments for high-resolution studies of the electronic and chemical structure of functional nano-materials and catalytic interfaces, with a focus on measurements under operando and/or ambient conditions: (i) a versatile solid-state spectroscopy setup with optional wide-angle lens and in-situ electrical characterization, (ii) a HAXPEEM instrument for sub-µm spectro-microscopy applications, (iii) an ambient pressure system (> 1 bar) for operando studies of catalytic reactions and (iv) a time-of-flight spectrometer as a full-field k-microscope for measurements of the 4D spectral function ρ(EB,k). The X-ray optics were designed to deliver high brightness photon flux within the HAXPES energy range 2.4 – 15 keV. An LN2-cooled double-crystal monochromator with interchangeable pairs of Si(111) and (311) crystals is optionally combined with a double channel-cut post-monochromator to generate X-rays with variable energy bandpass adapted to the needs of the experiment. Additionally, the beam polarization can be varied using a diamond phase plate integrated into the beamline. Adaptive beam focusing is realized by Be compound refractive lenses and/or horizontally deflecting mirrors down to a spot size of ∼20x17 µm2 with a flux of up to 1.1x1013 ph/s (for Si(111) at 6 keV).
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| 8. |
- Tusche, C., et al.
(författare)
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Multi-MHz time-of-flight electronic bandstructure imaging of graphene on Ir(111)
- 2016
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 108:26
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Tidskriftsartikel (refereegranskat)abstract
- In the quest for detailed spectroscopic insight into the electronic structure at solid surfaces in a large momentum range, we have developed an advanced experimental approach. It combines the 3D detection scheme of a time-of-flight momentum microscope with an optimized filling pattern of the BESSY II storage ring. Here, comprehensive data sets covering the full surface Brillouin zone have been used to study faint substrate-film hybridization effects in the electronic structure of graphene on Ir(111), revealed by a pronounced linear dichroism in angular distribution. The method paves the way to 3D electronic bandmapping with unprecedented data recording efficiency.
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